1. Field of the Invention
The present invention relates to an automatic focusing device for use in a camera, capable of performing automatic focusing through measuring the light coming through the shooting lens of the camera and determining the amount of defocus of the image-forming plane from a predetermined focal plane.
2. Related Background Art
A focus detecting system has been known in which the amount of deviation (referred to as defocus amount .DELTA.Bf) of the plane on which the image of an object is actually formed from a predetermined focal plane of the shooting lens system is detected by measuring the light coming through the shooting lens system. This type of focus detecting system, generally referred to as a TTL system, is used extensively in cameras having an automatic focusing function, in which lens driving means such as a motor is controlled in accordance with the defocus amount .DELTA.Bf, thus performing the automatic focusing operation.
The relationship between the amount .DELTA.x of driving of the lens necessary for the focusing and the defocus amount .DELTA.Bf is represented by the following formula (1). EQU .DELTA.x=1/K.DELTA.Bf (1)
where, K represents a coefficient which represents the relationship between the lens driving amount .DELTA.x and the defocus amount .DELTA.Bf. This coefficient will be referred to as conversion coefficient K, hereinafter.
The conversion coefficient K varies depending on factors such as the focal distance of the lens, particularly the power arrangement of the lens system and the distance R which is the distance between the object point and the predetermined focal plane.
The distance R and the lens power arrangement are changed, respectively each time the object is changed and each time the lens assembly is changed. Therefore, the conversion coefficient K must be calculated each time the focusing operation is conducted and each time the lens assembly is changed. Actually, however, it is quite troublesome to conduct such a computation, because a voluminous computation has to be conducted for calculating the conversion coefficient K from the lens power arrangement and the distance R and because the distance R itself cannot be definitely obtained until the object is exactly focused on. Thus, it is very difficult to conduct such a calculation by a microprocessor or a similar computing means incorporated in the automatic focusing device. For these reasons, it has been a common measure to previously set the values of the conversion coefficient K.
To cope with these problems, in the case of an automatic focusing device in apparatus having interchangeable lenses, e.g., single-lens reflex cameras, each interchangeable lens is provided with its own conversion coefficient K which suits the property of the lens. In this regard, U.S. Pat. No. 4,509,842 discloses a zoom lens system in which the focal distance range is divided into a plurality of regions and a value of conversion coefficient K is set beforehand for each of the regions of the focal distance range. In conventional automatic focusing devices, the previously set value of the conversion coefficient K is applied to the formula 1 so that the lens driving amount .DELTA.x is derived from formula (1) for each region of the focal distance range. Thus, in the conventional systems, the conversion coefficient K given for each region of the focal distance range is a constant value. Actually, however, the value of the conversion coefficient K varies depending not only on the focal distance range but also on other factors. Thus, the conventional automatic focusing systems which rely upon constant values of the coefficient K in solving the formula (1) cannot provide a high degree of precision of the focusing operation.
The representation of the relationship between the defocus amount and the lens driving amount employing the constant value of the conversion coefficient K is extremely difficult particularly in lens systems of the type in which the focusing is effected by moving element lenses of a front lens group as in most zoom lens systems.
FIGS. 1A and 1B show the manner in which the value of the conversion coefficient K varies in accordance with changes in the distance R and the lens arrangement in a lens system in which the focusing operation is conducted by moving element lenses of a front group. More specifically, FIG. 1A shows by solid lines L light paths for a focusing lens 1a which is focused on infinity. Broken lines S in FIG. 1A show the defocus amount .DELTA.Bf.sub.1 which is effected when an object O in a short distance is viewed through the focusing lens 1a in the state explained above. On the other hand, FIG. 1B shows by solid lines S light paths for the focusing lens 1a which is focused in the short distance. Broken lines L in FIG. 1B show the defocus amount .DELTA.Bf.sub.2 which is effected when an object at infinity is viewed through the focusing lens 1a in the state explained above.
The lens driving amount necessary for focusing on the object O from the state shown in FIG. 1A and the lens driving amount necessary for focusing on the object at infinity from the state shown in FIG. 1B are as follows.
(a) from infinity to object O in short distance: PA1 (b) from object O to infinity:
+X.sub.1 PA2 -X.sub.2
It will be seen that the defocus amounts in the states shown in FIG. 1A and FIG. 1B are different from each other as shown below, even though the absolute values of the lens driving amounts are equal. EQU .vertline..DELTA.Bf.sub.1 .vertline..noteq..vertline..DELTA.Bf.sub.2 .vertline.
In other words, the same defocus amount .vertline..DELTA.Bf.vertline. does not always provide the same lens driving amount .DELTA.x necessary for focusing. This fact clearly shows that the value of the conversion coefficient K is not constant. Thus, the conventional automatic focusing device which relies upon an assumption that the conversion coefficient K is constant suffers from the following problems.
Namely, driving of the lens by the calculated lens driving amount .DELTA.x may cause the lens to move beyond the focusing point or to stop before reaching the focusing point. It is thus impossible to position the lens correctly at the focusing point by a single cycle of focusing operation: namely, the object is correctly focused on only after several repeated cycles of the focusing operation, resulting in a too low focusing speed and intermittent non-smooth driving of the lens.